In the field of solar thermal power generation and the like in which rays of sunlight are condensed and used as energy, a condensation rate of a heliostat 3 (a reflecting mirror) for condensing rays of sunlight is raised by use of a mirror having a concave surface. It is desirable that this concave surface should be formed in three-dimensions with a toroid included therein. A durable mirror having a concave surface has been proposed (see Patent Document 1, for instance).
However, its manufacturing costs are high. In addition, it is difficult to secure the manufacturing accuracy, and to manufacture the mirror in a large size. For this reason, used is a heliostat including small-sized and flat reflecting mirrors (facets 31) made coincide with a toroid 53 in a pseudo manner as shown in FIG. 5.
To put it specifically, as shown in FIG. 5, multiple facets 31 are adjusted and fixed in such a way as to coincide with a form representing the toroid 53, for instance, a form representing a spherical surface. In this case, the adjustment needs to be repeated frequently, because the accuracy of the mounting positions of the respective facets 31 is low.
Patent Document 1: Japanese patent application Kokai publication No. 2002-154179